This invention relates generally to compositions and methods for the formation of protective, corrosion-inhibiting rinses and seals for use to impart additional corrosion resistance to structural materials without the use of chromium in the hexavalent oxidation state. More particularly, this invention relates to non-toxic, corrosion-protective rinses and seals for metal phosphating, anodizing, and “black oxiding” processes based on tetravalent cerium, praseodymium, or terbium and methods of making and using the same.
Metals like aluminum, zinc, titanium, iron, cadmium, tin, indium, lithium, beryllium, magnesium, niobium, tantalum, zirconium, lead, rare earths, copper, and silver, their alloys, or items plated with these metals, require protection from corrosion due to their low oxidation-reduction (redox) potentials or ease of oxide formation. These metal alloys have many uses that range from architectural adornments, to protective coatings themselves, to automotive, structural aerospace, and electronic components, to name a few. The unalloyed metals typically form an outer layer of natural oxide: a “passive film” that serves to protect them and reduce their overall rate of corrosion. However, the corrosion protection offered by the naturally formed oxide layer on certain alloys of these metals is not complete and corrosion will eventually occur unless some form of additional corrosion protection is used. Thus, for example, steels are typically “phosphated” to provide an impermeable coating that not only resists corrosive attack, but also provides a paint base. Additionally, architectural and structural aluminum are frequently “anodized” to form an impermeable oxide film for the same reasons.
Inhibiting the initiation, growth, and extent of corrosion is a significant part of component and systems design for the successful long-term use of metal objects. Uniform physical performance and safety margins of a part, a component, or an entire system can be compromised by corrosion.
One method of enhancing the corrosion resistance of these alloys includes the use of a chemically- or electrolytically-generated coating such as an anodized coating (typically on aluminum), a phosphate coating (typically on electrogalvanized or bare steel), or a black oxide coating (for high strength bearing and tool steels). The metal is exposed to a compound that chemically alters the surface (in phosphating and black oxiding) or an electric current (in anodizing) and forms a coating that provides some corrosion resistance by forming a barrier film. The morphology and possibly the chemistry of the anodic coating or phosphate coating can allow for the formation of a strong bond with subsequently-applied paint systems. An anodic coating is usually applied via immersion in an electrolytic cell. A phosphating or black oxide solution may be applied by immersion, by spray, or by manual means.
These coatings frequently exhibit “flaws” such as pores, pinholes, or thin portions in the coating after formation and do not contain any inherent means to “repair” these coating breaches. The application of a second solution is necessary to fill the pores in the coating and deposit compounds that will act as long-term corrosion protective species. These “second solutions” are termed “rinses” or “seals” in the corrosion literature. The term “rinse” is typically used for the second solution applied to phosphating and black oxide coatings, whereas the term “seal” usually refers to the second solution applied to anodic coatings. These rinses and seals are typically applied via spray techniques, but immersion, fogging, and wiping are also accepted practices.
Hexavalent chromium has traditionally been the active corrosion-inhibiting agent used in rinses and seals for the formation of protective coatings for iron, electrogalvanized iron, aluminum, zinc, magnesium, titanium, cadmium, tin, indium, lithium, and their alloys. Niobium, tantalum, zirconium, beryllium, lead, rare earths, copper, and silver may also be treated with hexavalent chromium rinses and seals for special applications. The three main coating processes that use these rinses and seals are 1) the phosphating process for steel and galvanized steel products, 2) the anodization process for a host of structural metals, and 3) the black oxide process for high-strength steel and iron used for bearing materials. Table 1 illustrates the processes that typically utilize a final chrome “rinse” or “seal” to impart additional corrosion protection to a given substrate material.
TABLE 1Current Rinse and Seal Processes Using Hexavalent ChromiumComments/Government/ProcessExamplesSubstrate MetalsASTM/Mil SpecsRinses for zincUsed as a paint baseZinc-coated steel,MIL-P-50002phosphating onon all automotivezinc, or bare steel areDoD-P-16232steel, steel products,bodies, also for someusual substrates.MIL-HDBK-205and nonferrouscoil and sheet stock.Also for aluminum,SAE-AMS2481alloysUsed as a lubricatingmagnesium, copper,QQ-P-416layer on tooling dies.titanium, cadmium,and silver in lesscommon applications.Seals for anodizedUsed extensively forAluminum andMIL-A-8625aluminum includingarchitectural andaluminum alloysSAE-AMS2470sulfuric, chromic,decorativeASTM B580oxalic, boric,applications, adhesiveASTM D1730sulfonated organicbonding, siding, etc.AA46-78acids, citric, andAlso used as a paintphosphoric acidbase.anodizingRinses for ironUsed as a paint baseSteel and iron alloysTT-C-490phosphating on bareon coil coatings forMIL-HDBK-205steelsgeneral appliance andSAE-AMS2481siding applications.QQ-P-416Different from Zn andMn phosphating.Rinses forUsed solely as a solidMostly bare steel.MIL-P-50002manganeselubricant, not as aCan also be used onDoD-P-16232phosphating onpaint base. Usedhigh-strength copperMIL-HDBK-205steel and steelextensively on bearingalloys.SAE-AMS2481alloys, also onmaterials.nonferrous alloysRinses for “blackUsed solely as a solidMostly bare steel.MIL-C-13924oxide” and otherlubricant, not as aCan also be used onMIL-C-46110oxide lubricatingpaint base. Usedhigh-strength copperSAE-AMS2485layersextensively on bearingalloys.materials.Seals for anodizedUsed as a paint andMagnesium andMIL-M-45202magnesiumadhesive base.magnesium alloysASTM D1732including sulfuric,SAE-AMS2475chromic, oxalic,MIL-C-13335boric, sulfonatedorganic acids, citric,and phosphoric acidanodizingSeals for anodizedUsed as a paint andTitanium andSAE-AS4194titanium includingadhesive base.titanium alloysSAE AMS-2488sulfuric, chromic,oxalic, boric, citric,hydrofluoric, andphosphoric acidanodizingSeals for anodizedUsed as a paint andZinc and zinc alloysMIL-A-81801zinc includingadhesive base.sulfuric, chromic,oxalic, boric,sulfonated organicacids, citric, andphosphoric acidanodizingSeals for anodizedUsed as a paint andIron, steel, and steelQQ-P-35steel includingadhesive base.alloyssulfuric, chromic,oxalic, boric, andphosphoric acidanodizingSeals for anodizedUsed for a number ofCopper, cadmium,QQ-P-416copper, cadmium,applications,silver, tantalum,silver, tantalum,principally as a paintniobium, zirconium,lead, cobalt,and adhesive base.tin, indium,niobium, zirconium,For example, niobiummanganese and theirtin, indium, andand tantalumalloysmanganesecapacitors, cadmiumincluding sulfuric,plate, silver solder,chromic, oxalic,and zirconium forboric, sulfonatednuclear applications.organic acids, citric,and phosphoric acidanodizing
As shown in Table 1 above, there are three “generic” phosphating processes for steel and steel alloys—zinc, manganese, and iron phosphating. Differences in the coating solutions result in different chemistries and physical attributes in the formed coatings. For example, zinc phosphating is used primarily on galvanized steel sheet, and results in an ideal surface morphology for paint adhesion if the crystals are small in size, and as a solid lubricant for larger size crystals. Manganese phosphating, however, results in a hard, lubricious coating that has no use as a paint base, but exhibits excellent characteristics as a solid lubricant. Manganese phosphating coatings are rarely subjected to a post-chrome rinse, because the corrosion resistance of these coatings is of lesser concern. Iron phosphating is also used as a paint and adhesive base, and always receives post-treatments for corrosion protection.
Similar differences are also noted in anodizing processes. Anodizing processes involve the application of an electric potential under a variety of acidic conditions to the substrate to be coated. Sulfuric acid is the conventional anodizing acid used to form hard oxide films on aluminum, although other anodization solutions have specialized applications. For example, phosphoric acid may be used for adhesive bonding applications on aluminum. Oxalic acid anodization results in a harder, denser coating with higher corrosion resistance than sulfuric acid anodization and is used more often in Europe. Boric acid anodization is used frequently for electronic capacitors although citric and tartaric acid anodization can be used for the same application. Anodization with sulfonated organic acids (such as sulfosalicylic or sulfophthalic acids) is used to impart color during the anodization process. Chromic acid anodization is used on parts with complex shapes where final sealing or rinsing is not possible. Other acids, including hydrofluoric acid, have been used for special applications or in proprietary formulations. Those skilled in the anodization art know that a wide variety of anodizing processes exist due to the multitude of substrate metals, anodizing acids, applied voltages, and final applications.
Finally, “black oxide” coatings are applied to high strength steels and copper-containing alloys to impart a lubricious coating. The difference between “black oxide” coatings and other lubricious coating processes (such as manganese phosphating) is that “black oxide” coatings are applied under caustic, elevated temperature conditions. For example, a concentrated sodium hydroxide solution is raised to its boiling point and the substrate metal is then immersed in this solution. This results in the formation of a lubricious coating of magnetite/ferrite on the surface of steel alloys.
Other coating processes that result in coatings with no inherent self-healing characteristics have also been enhanced through the use of hexavalent chromium rinses and seals. Carbonate coatings on metals such as zinc, iron, magnesium, and especially copper have been described in the early literature as providing some degree of corrosion protection. These coatings can be further enhanced through the use of hexavalent chromium rinses to deposit inhibiting compounds to self-heal coating breaches. Other oxide, phosphate, oxalate, silicate, aluminate, borate or polymeric coatings, or combinations thereof, can also be enhanced via hexavalent chromium rinses and seals.
For each of these three generic coating processes (phosphating, anodizing, and black oxiding), a second, subsequent chemical treatment is often applied. The nature of this second treatment is dependent upon the desired final characteristics of the metal piece. For phosphating and black oxiding processes, this second treatment is usually a rinse of hexavalent chromium, to impart additional corrosion protection to the coating. For anodizing processes, the second treatment can impart a number of useful attributes to the work piece. This second “sealing” process for anodized coatings can include: 1) pure boiling water (to plug the pores with a hydrated alumina composition); 2) silicates (to plug the pores with a silicate composition); 3) dyes or metal-dye complexes (to impart color to the anodic coating); 4) metal salts followed by cathodic reduction (to color the coating via the formation of metals or metal sulfides in the pores); 5) lubricating additives such as molybdenum disulfide or dispersions of polytetrafluoroethylene (to fill the pores with a lubricious additive); and 6) hexavalent chromium seals to fill the pores with chromate species. It is noteworthy that the only one of these six generic sealing processes that results in a coating with self-healing characteristics is the hexavalent chromium seal. The other sealing processes for anodic coatings may temporarily increase the corrosion resistance of the coating by plugging the pores in the oxide coating (e.g., with hydrated alumina or silicate), but the coating does not retain any corrosion-inhibitive species.
The various coating processes to which the art described in this invention is applicable are shown in Table 1 above. The frequent use of hexavalent chrome to “rinse” or “seal” the coating (phosphate, anodic, or black oxide) formed in the first unit operation of the process to impart additional corrosion resistance connects them. These solutions are usually simple formulations consisting of nothing more than dissolved chromium trioxide, chromate, or dichromate. These formulations are usually applied by spraying, although immersion, fogging, or even wiping may also be used.
Sometimes these hexavalent chromium rinse or sealing formulations will contain other constituents. Some formulations include minor concentrations of fluorides. These fluorides act to “etch back” the coating formed in the first unit operation (e.g., phosphate, anodic, or black oxide), thus further facilitating the deposition of corrosion-inhibiting species. Rinsing solutions for phosphate solutions are frequently observed to include phosphoric acid in addition to hexavalent chromium in order to reduce staining of the phosphate coating by the hexavalent chromium. These hexavalent chromium rinse or sealing solutions can also contain other constituents, such as ferricyanides or molybdates. The presence of these other constituents is significant in light of the chemistry developed and presented herein.
Significant efforts have been made to replace chromium with other metals for corrosion-inhibiting applications due to toxicity, environmental, and regulatory concerns. Cerium is one non-toxic, non-regulated metal that has been considered as a chromium replacement. Cerium (like chromium) exhibits more than one oxidation state (Ce+3 and Ce+4). In addition, the oxidation-reduction potential of the Ce+4—Ce+3 couple is comparable to the Cr+6—Cr+3 couple. For example, in acid solution:Ce+4+eCe+3 +1.72 VCr+6+3e−Cr+3 +1.36 VPraseodymium and terbium also exhibit more than one oxidation state (Pr+3 and Pr+4, Tb+3 and Tb+4). Tetravalent praseodymium and terbium are even stronger oxidizing agents than cerium (with calculated redox potentials of +3.2 V in acidic solution—Nugent, L. J., et al., J. Inorg. Nucl. Chem. 33:2503-30, 1971):Pr+4+e−Pr+3 +3.2 VTb+4+e−Tb+3 +3.2 VCr+6+3e−Cr+3 +1.36 VAccordingly, several processes have been reported in the literature, which make use of cerium in rinsing or sealing bath solutions. However, the coatings formed by these processes provide only limited corrosion protection and do not approach the benefit derived from the use of hexavalent chromium. None of the prior art recognizes the need to “valence stabilize” tetravalent cerium to ensure its long-term stability, nor the need to form tetravalent cerium compounds of optimum solubility characteristics.
The use of film-forming substances, such as polymers, silicates, sol-gel, etc., which have no inherent oxidizing character in sealing or rinsing coating solutions, has been described in the literature. The film formers may enhance short-term corrosion resistance by functioning as a barrier layer. Barrier layers lacking an active corrosion inhibitor have been demonstrated to be capable of inhibiting corrosion as long as the barrier is not breached, as by a scratch or other flaw. Film formers can actually enhance corrosion on a surface after failure due to the well known effects of crevice corrosion.
1) Rinses for Phosphate Coatings
U.S. Pat. No. 2,790,740 to Ayres et al. describes the use of a tetravalent cerium compound (i.e., ceric sulfate) as an accelerator for phosphate coatings on aluminum and zinc. The cerium is added simultaneously with the phosphate treatment. No provisions for post-treatment of the formed phosphate coating through an additional rinse are described. Pores formed during the phosphating step are therefore not sealed. This patent also describes the need to incorporate zinc or manganese compounds with the cerium, as cerium appears to be effective only when used in the presence of substantial proportions of zinc or manganese.
U.S. Pat. No. 2,698,266 to Thirsk decribes the use of hexavalent chromium/tetravalent cerium rinses to seal phosphate and arsenate coatings on aluminum. The use of hexavalent chromium in conjunction with tetravalent cerium represents no appreciable reduction in bath toxicity.
German Patent No. DE 40 41 091 A1 to Metallgesellschaft AG describes the use of trivalent cerium along with tetravalent cerium in a 2:1 to 9:1 ratio for the passivating of phosphated coatings on steel and aluminum. These coating solutions also incorporate fluoride, carboxylate, hydroxycarboxylate, aminocarboxylate, molybdate, and/or tungstate ions in the solution. However, the importance of tetravalent cerium within the formed coating, the “valence stabilization” of this ion, and the solubility ranges for formed tetravalent cerium compounds are not described.
2) Seals for Anodic Coatings
U.S. Pat. No. 5,192,374 to Kindler describes the formation of an aluminum oxide (boehmite) coating on structural aluminum, followed by treatment with a soluble cerium salt and a metal nitrate at 70° C. to 100° C. to form cerium oxides and hydroxides for increased corrosion resistance. The formed oxides and hydroxides are described as filling the pores in the boehmite coating. Also, Stoffer et al. in U.S. Pat. No. 5,932,083 describe the use of a solution containing cerium and an oxidizing agent for treatment of aluminum alloys. The aluminum-containing substrate is electrolyzed in this solution, forming a mixed aluminum oxide/cerium oxide (or hydrated cerium oxide) coating on the aluminum as a barrier film. The formation of tetravalent or hydrated tetravalent cerium oxide is described. However, neither Kindler nor Stoffer et al. teach the use of “valence stabilizers”, which are important for use of tetravalent cerium compounds having aqueous solubilities that are sufficiently high to ensure long-term self healing of the coating. The cerium oxides and hydrated oxides described in these patents function merely as pore-filling barrier layers, and not as active self-healing inhibitors within the coating. Further, the use of tetravalent cerium oxides and hydroxides as corrosion inhibitors results in lower corrosion performance, as is described herein, due to the fact the electrostatic double layers around these species are much smaller than those exhibited by tetravalent cerium species containing 50% or less oxide or hydroxide as attached ligands.
U.S. Pat. Nos. 5,635,084; 5,582,654; and 5,194,138, all to Mansfield et al., describe methods for treating the surface of an aluminum alloy having a relatively high copper content, so as to make the surface resistant to corrosion. The method comprises: a) removing substantially all of the copper from the surface of the alloy, b) contacting the surface with a first solution containing cerium, c) electrically charging the surface while contacting with an aqueous molybdate solution, and d) contacting the surface with a second solution containing cerium. U.S. Pat. No. 5,756,218 to Buchheit et al. describes a process for the corrosion protection of metallic materials that includes sealing a coating with an aqueous solution consisting essentially of at least one soluble metal salt (i.e., Ce). However, the '084, '654 and '218 patents make use of h exavalent chromium in the coating process, and so no advantage in toxicity reduction is achieved. Moreover, electrolysis will only oxidize cerium to the tetravalent state in the outer regions of the already-formed cerium-containing coating. The importance of tetravalent cerium and the functional parameters for tetravalent cerium-containing complexes are not described in any of these prior art references.
U.S. Pat. No. 6,022,425 to Nelson et al. describes the application of a corrosion-resistant coating for aluminum based on cerium, which cerium is oxidized to the tetravalent oxidation state, resulting in the formation of tetravalent or hydrated cerium oxides. However, these references teach tetravalent cerium compounds having aqueous solubilities that are so low they function as barrier films or sealants, rather than active corrosion inhibitors. Moreover, the use of valence stabilizers for forming complexes with tetravalent cerium is not disclosed.
European Application No. EP 0 902 103 A1 by Nippon Steel Corporation describes the application of a trivalent cerium solution with organic oxoacids to aluminum or galvanized steel. U.S. Pat. No. 6,200,672 B1 to Tadokoro et al. describes the use of rare earth and/or Group IVA solutions with selected organic molecules for treatment of metal surfaces. U.S. Pat. No. 5,964,928 to Tomlinson describes the use of a Group IVA compound (i.e., zirconium, titanium, or hafnium) in combination with a rare earth element and optionally a fluoride. European Application No. EP 0 839 931 A2 by Nihon et al. describes an aqueous, metallic surface treating solution comprising a metal element including Ce, an oxidizing source, and an oxyacid or oxyacid salt of phosphorus or an anhydride thereof. However, none of these references teach the presence of a valence stabilized, oxidized rare earth element such as cerium, praseodymium, or terbium in the formed seal, whose availability to the corroding system is controlled via the solubility of the oxidized rare earth compounds. In order to function as a true replacement for hexavalent chromium, which is itself a highly oxidized species, the rare earth compound must be oxidized in the formed seal.
U.S. Pat. No. 6,206,982 B1 to Hughes et al. describes the use of a four component system to provide corrosion protection of aluminum. One of these components includes a rare earth compound, especially cerium.
The use of colloidal suspensions of tetravalent cerium oxide (CeO2) in anticorrosive coatings is described in U.S. Pat. Nos. 5,922,330 and 5,733,361 to Chane-Ching et al.; PCT International Publication No. WO 96/26255 by Rhone Poulenc Chimie; and PCT International Publication Nos. WO 01/36331 A1 and WO 01/38225 A1 by Rhodia Terres Rares. The CeO2 exhibits a solubility that is too low for effective release of corrosion-inhibiting tetravalent cerium ions.
An aqueous dispersion of a cerium compound with other rare earths, transition metals, aluminum, gallium, or zirconium is described for anticorrosive agents in PCT International Publication No. WO 01/55029 A1 by Rhodia Terres Rares. Similarly, an aqueous dispersion of cerium oxide in combination with additives such as beta-diketones, alpha-hydroxycarboxylic acids, beta-hydroxycarboxylic acids, or diols is described for anticorrosive agents in U.S. Pat. No. 6,033,677 to Cabane et al. Neither of these references define the need for cerium to be in the tetravalent oxidation state to achieve anticorrosive effects.
The following U.S. patents and published applications provide further examples of corrosion-inhibiting seals from metallic surfaces: U.S. Pat. No. 6,248,184 B1 to Dull et al.; U.S. Application Publication No. 2002/0003093 A1 by Dull et al.; U.S. Application Publication No. 2003/0019391 A1 by Kendig; U.S. Application Publication No. 2003/0024432 A1 by Chung et al.; U.S. Application Publication No. 2002/0033208 A1 to Krishnaswamy, Jr.; U.S. Pat. No. 6,451,443 B1 to Daech; and U.S. Pat. No. 6,299,983 B1 to Van Alsten. However, none of these references teach the need for at least one rare earth element to be in the tetravalent oxidation state.
Accordingly, the need remains for improved rinses and seals which have an effectiveness, ease of application, and performance comparable to coatings formed with hexavalent chromium and which do so without the use of toxic or currently regulated materials.